U.S. patent number 6,388,674 [Application Number 09/315,978] was granted by the patent office on 2002-05-14 for gamut mapping method and apparatus.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Masahiko Ito, Naoya Kato.
United States Patent |
6,388,674 |
Ito , et al. |
May 14, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Gamut mapping method and apparatus
Abstract
When an output color gamut is different from an input color
gamut in a picture input/output system such as DTP, the color is
corrected with a predetermined function for a difference in the
lightness-directional dynamic range. A three-dimensional
compression is made to compress the color of areas C and D to an
area (A+B), and next a two-dimensional compression (or shrinkage)
is made to compress the color of the area (B+C) to the area B, for
example.
Inventors: |
Ito; Masahiko (Chiba,
JP), Kato; Naoya (Chiba, JP) |
Assignee: |
Sony Corporation (Tokyo,
JP)
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Family
ID: |
15445965 |
Appl.
No.: |
09/315,978 |
Filed: |
May 21, 1999 |
Foreign Application Priority Data
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May 28, 1998 [JP] |
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10-148131 |
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Current U.S.
Class: |
345/590; 345/589;
382/167; 345/591; 358/519; 358/520; 382/166; 345/600 |
Current CPC
Class: |
H04N
1/6058 (20130101); G09G 5/04 (20130101) |
Current International
Class: |
H04N
1/60 (20060101); G09G 005/04 () |
Field of
Search: |
;345/150,129,130,127,133,154,425,426,431,186,507,509,199,202,6,22,83,84,589-591
;358/1.9,502,504,518,519-520 ;382/162,167,166 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 684 728 |
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Nov 1995 |
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EP |
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0 828 381 |
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Mar 1998 |
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EP |
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Other References
M Ito and N. Katoh: "Gamut Compression for Computer Graphic Images"
Extended Abstract SPSTJ 70.sup.th Aniverary Symp. Fine Imaging,
1995, pp. 85-88. .
Katoh and M Ito N: "Three-dimensional Gamut Mapping Using Various
Color Difference Formulae and Color Spaces" Journal of Electronic
Imaging, SPIE + IS&T vol. 8, No. 4, Oct. 1999 pp.
365-378..
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Primary Examiner: Luu; Matthew
Assistant Examiner: Sajous; Wesner
Attorney, Agent or Firm: Kananen, Esq.; Ronald P. Rader,
Fishman & Grauer, PLLC
Claims
What is claimed is:
1. A color mapping method of changing a color, when an output color
gamut is different from an input color gamut, the input color gamut
to the output color gamut, comprising:
correcting the color using a predetermined function for a
difference in the lightness-directional dynamic range, and
correcting the color by a combination of a three-dimensional
compression of lightness, chroma and hue and a two-dimensional
shrinkage or expansion of the lightness and chroma, wherein
correcting the color further includes:
dividing the input gamut by four with a first straight line passing
through a minimum value point L*min of the lightness L* in the
output gamut and a second straight line passing through a maximum
value point L*max of the lightness L* in the output gamut, these
first and second straight lines intersecting each other at a point
(C*th, L*_th) of the lightness value L*_th having a maximum chroma
value C*_max of the output gamut in a two-dimensional plane of the
lightness L* and chroma C* with the hue h kept constant, thereby
defining an area A above the first straight line and below the
second straight line, an input area B other than the area A and
corresponding to the output color gamut, an input area C other than
an area corresponding to the output color gamut, and an area D
other than the output and input color gamuts; and
leaving the color in the area A as it is and correcting the color
by a combination of the three-dimensional compression and
two-dimensional shrinkage or expansion.
2. The method as set forth in claim 1, further including correcting
the color with an exponential function for a difference in the
lightness-directional dynamic range.
3. The method as set forth in claim 1, wherein correcting the color
by the two-dimensional shrinkage or expansion occurs after the
three-dimensional compression.
4. The method as set forth in claim 1, wherein correcting the color
by the three-dimensional compression occurs after the
two-dimensional expansion.
5. The method as set forth in claim 4, further including subjecting
the colors of the areas C and D to the three-dimensional
compression to the areas A and B, while subjecting the colors of
the areas B and C to the two-dimensional shrinkage to the area
B.
6. The method as set forth in claim 4, further including subjecting
the colors of the areas B, C and D to the three-dimensional
compression to the area A, while subjecting the color of the area B
to the two-dimensional compression to the areas B and C.
7. The method as set forth in claim 4, further including subjecting
the color of the area B to the two-dimensional expansion, while
subjecting the colors of the areas D and C to the three-dimensional
compression to the areas A and B.
8. The method as set forth in claim 4, further including subjecting
the color of the area B to the two-dimensional expansion to the
areas B and C, while subjecting the color of the area D to the
three-dimensional compression to the areas A, B and C.
9. The apparatus as set forth in claim 1, wherein said
predetermined function is an exponential function.
10. The apparatus as set forth in claim 1, wherein said means
causes the two-dimensional shrinkage or expansion to occur after
the three-dimensional compression.
11. The apparatus as set forth in claim 1, wherein said means
causes the three-dimensional compression to occur after the
two-dimensional expansion.
12. A color mapping apparatus comprising:
a color mapping means for changing, when an output color gamut is
different from an input color gamut, a color in the input color
gamut to one in the output color gamut by using a color mapping
table, said color mapping table created by using a predetermined
function for a difference in the lightness-directional dynamic
range, and then combining a three-dimensional compression of
lightness, chroma and hue and a two-dimensional shrinkage or
expansion of the lightness and chroma, wherein said means includes
means for dividing the input gamut by four with a first straight
line passing through a minimum value point L*min of the lightness
L* in the output gamut and a second straight line passing through a
maximum value point L*max of the lightness L* in the output gamut,
these first and second straight lines intersecting each other at a
point (C*th, L*_th) of the lightness value L*_th having a maximum
chroma value C*_max of the output gamut in a two-dimensional plane
of the lightness L* and chroma C* with the hue kept constant,
thereby defining an area A above the first straight line and below
the second straight line, an input area B other than the area A and
corresponding to the output color gamut, an input area C other than
an area corresponding to the output color gamut, and an area D
other than the output and input color gamuts.
13. The apparatus as set forth in claim 12, wherein said means
further leaves the color in the area A as it is and corrects the
color by a combination of the three-dimensional compression and
two-dimensional shrinkage or expansion.
14. The apparatus as set forth in claim 13, wherein said means
further subjects the colors of the area C and D to the
three-dimensional compression to the areas A and B, while
subjecting the colors of the areas B and C to the two-dimensional
shrinkage to the area B.
15. The apparatus as set forth in claim 13, wherein said means
further subjects the colors of the areas B, C and D to the
three-dimensional compression to the area A, while subjecting the
color of the area B to the two-dimensional compression to the areas
B and C.
16. The apparatus as set forth in claim 13, wherein said means
subjects the color of area B to the two-dimensional expansion,
while subjecting the colors of the area D and C to the
three-dimensional compression to the areas A and B.
17. The apparatus as set forth in claim 13, wherein said means
subjects the color of the area B to the two-dimensional expansion
to the areas B and C, while the color of the area D is subjected to
the three-dimensional compression to the areas A, B and C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gamut mapping method usable when
a color reproduction range (referred to as "color gamut"
hereinunder) of an output system is different from that of an input
system suitable for use to reproduce by an electronic device of one
kind of a color picture data supplied from an electronic device of
another kind, both the devices included in a picture input/output
system such as a desk top publishing (DTP). The gamut mapping
method and apparatus are suitably used in hard copying, by a
printer having a small color gamut, of a color picture signal
supplied from a monitor whose color gamut is large, for
example.
2. Description of Related Art
Recently, with electronic devices having remarkably been innovated
for lower prices and higher speeds, color desk top publishing
(color DTP) network, internet network, and so forth have been
prevailing and a variety of electronic devices dealing with color
picture signals (referred to simply as "device" hereinunder) has
been incorporated in such network systems. Thus, currently, for a
printer to print out a color picture signal supplied from a
monitor, for example, it is necessary to introduce the so-called
device-independent color (DIC) concept that a color picture from a
device of one kind is reproduced in a same color also at a device
of another kind, both the devices being included in a picture
input/output system.
A system to implement a DIC is generally called a "color management
system (CMS)". In the CMS, measured physical values of color
signals at an input device are adjusted to those at an output
device to implement a DIC. Referring now to FIG. 1, there is
schematically illustrated the color management system (CMS) by way
of example. The CMS comprises devices such as a video camera 61,
monitor 62, printer 63, and so forth. In this CMS, since a color
signal of an input or output picture is dependent upon each of the
devices, it is necessary to adjust measured physical values of a
color signal at the video camera 61 or monitor 62 as an input
device to those of a color signal at the printer 63 as an output
device.
In the CMS shown in FIG. 1 for example, since a color signal of a
picture on the monitor 62 as an input device is an RGB color signal
dependent upon the device, an input device profile (monitor
profile) created using a predetermined transform formula or table
is used to transform the color signal to one independent of the
device and further an output device profile (printer profile) is
used to transform the device-independent color signal to a one such
as CMYK or the like dependent upon the printer 63 as an output
device, thereby printing out the picture from the printer 63, as
shown in FIG. 2.
Namely, when a color signal is transformed to an output color
signal in the CMS, a transform formula or table called "device
profile (will be referred to simply as "profile" hereinunder as the
case may be) is used to transform the input color signal to a color
signal in a color space independent of each device (CIE/XYZ, and so
forth), thereby implementing the DIC. The "device profile" may be
considered as a file of parameter groups calculated based on a
relationship between a color signal (RGB, CMYK, and so forth) of a
device and a color measured (XYZ, L*a*b*, CIE/L*C*h, and so forth)
by a colorimeter or the like.
However, each input/output device is limited in color reproduction
range (gamut), namely, in a color gamut. The color gamut varies
greatly from one kind of device to another. Therefore, it has been
physically difficult to reproduce a completely same color at all of
the different kinds of devices, and especially, the difference in
color gamut from one to another kind of device has been a great
barrier against implementation of the CMS. This will further be
described concerning a computer graphic (CG) monitor and an inkjet
printer (referred to simply as "printer" hereinunder).
As well known, the CG monitor reproduces a color by the addition
mixture of primary colors emitted from three phosphors, red (R),
green (G) and blue (B). Therefore, the color gamut of the CG
monitor depends upon the kinds of phosphors used in the CG monitor.
On the other hand, the printer reproduces a color with inks of cyan
(C), magenta (M), yellow (Y) and black (K). The color gamut of the
printer varies from one to another kind of ink as well as from one
to another type of paper as a picture recording medium and from one
to another gradation reproducing method.
FIG. 3 shows the result of a comparison between a color gamut GMmon
of the CG monitor and a color gamut GMijp of the printer, each
obtained by integration in the direction of L* and plotting in a
plane of a*-b*. As seen, the color gamut GMijp of the printer is
smaller than the color gamut GMmon of the CG monitor. Especially,
the G (green) and B (blue) gamuts are very smaller. As seen from
FIG. 3, the peak chroma deviates in the direction of lightness also
in other areas in which the color gamuts are not so much different.
Therefore, when a color displayed on the CG monitor is reproduced
by the printer, it is physically different for the printer to
reproduce the color in the areas of a high lightness and chroma on
the CG monitor.
Thus, when the color gamut of an output device is smaller than that
of an input device, all the colors displayed on the input device
cannot be reproduced by the output device. Therefore, in such a
case, it is necessary to make some operation for compressing the
color gamut of the input device into that of the output device. At
this time, the color gamut of the input device have to be
compressed into that of the output device while maintaining picture
information (gradation, tone, and so forth) represented on the
input device as much as possible. Namely, the color should be
corrected to compress a color outside the color reproduction range
(color gamut) into the color gamut while maintaining an input
original picture information.
The operation to compress into the color gamut of the output device
a color which cannot physically be reproduced is generally called
"gamut compression". Taking in consideration a case that the color
gamut of the output device is larger than that of the input device,
the operation to transform the color gamut of the input device to
that of the output device of a different kind from the input device
will be referred to as "gamut mapping".
Since the color gamut of a printer as an output device is much
smaller than that of other input device, the color reproduction
depends greatly upon the method of gamut mapping in many cases. The
gamut mapping is done in a common color space not dependent upon
any device. It is most popular to effect the gamut mapping in a
CIE/L*C*h color space matching the human visual
characteristics.
The human eyes can perceive three attributes of a color, namely,
lightness, chroma and hue. The aforementioned CIE/L*C*h is a color
space based on these three attributes of color perceivable by the
human eyes. The CIE/L*C*h is a color space derived from an L*a*b*
color space by representing the latter in the font of spherical
coordinates in which L* indicates the lightness, C* indicates a
chroma and h indicates a hue. In the CIE/L*C*h color space, the
above three attributes may be handled as independent
parameters.
It is generally said that the gamut mapping should preferably be
done in a two-dimensional plane of the lightness L* and chroma C*
in the CIE/L*C*h space while the hue h is being maintained
constant. More particularly, the gamut mapping methods include a
chroma compression in which only the chroma C* is compressed while
the lightness L* and hue h are being kept constant as shown in FIG.
4, a lightness compression in which the lightness L* is compressed
in a direction of (L*, a*, b)=(50, 0, 0) while the hue h is being
kept constant as shown in FIG. 5, and other methods. Further, for a
gamut mapping by three-dimensional compression of the lightness,
chroma and hue h as well, it has been proposed to weight the three
color difference items (lightness, chroma and hue differences)
(referred to as "coefficient of compressibility" hereinunder) and
then map the lightness, chroma and hue in the direction of a
minimum color difference.
In the gamut mapping in which the lightness or chroma is compressed
with the hue kept constant, such as the lightness or chroma
compression, an emphasis has to be put on the compression in the
direction of lightness or chroma, which causes the following
problems:
If a compression is done in the direction of lightness L*, the
contrast is lowered and the entire picture is of less third
dimension. When a compression is done in the direction of chroma
C*, the picture becomes less vivid and impactful. Therefore, when
the gamut mapping is done with the hue kept constant, a picture
having a high chroma and third dimension such as a CG (computer
graphic) picture will lose its characteristic very much.
To prevent the above as much as possible, the compressions in the
direction of lightness L* and C* should be done at reduced ratios,
respectively, in a gamut mapping in which the hue h is somewhat
changed. To solve this problem, the Inventor of the present
invention has disclosed, in the Japanese Published Unexamined
Patent Application No. 08-238760, a gamut mapping method in which
coefficients of compressibility are assigned to the lightness,
chroma and hue differences, respectively, by weighting. This gamut
mapping method permits to compress the lightness L*, chroma C* and
hue h in a good balance. However, all data outside the gamut are
mapped over the gamut with a result that colors compressed in a
same direction are all mapped in a same color, so that they lose
the gradation.
SUMMARY OF THE INVENTION
Accordingly, the present invention has an object to overcome the
above-mentioned drawbacks of the prior art by providing a gamut
mapping method and apparatus adapted to attain a natural color
reproduction at different kinds of devices in a picture
input/output system such as DTP, and so forth.
The above object can be attained by providing a color mapping
method of changing, when an output color gamut is different from an
input color gamut, the input color gamut to the output color gamut
by correcting the color using a predetermined function for a
difference in the lightness-directional dynamic range and
correcting the color by a combination of a three-dimensional
compression of lightness, chroma and hue and a two-dimensional
shrinkage or expansion of the lightness and chroma.
In the color mapping method, the predetermined functions are used
for color correction against the difference in
lightness-directional dynamic range and the gradation in a low
lightness of a picture is maintained, thereby permitting to utilize
the output device color gamut to the maximum extent. The color
correction by the combination of the three-dimensional compression
of lightness, chroma and hue and two-dimensional shrinkage or
expansion of lightness and chroma, permits to maintain the
characteristics of the picture to the maximum extent.
The above object can also be attained by providing a color mapping
apparatus comprising a color mapping means for changing, when an
output color gamut is different from an input color gamut, a color
in the input color gamut to one in the output color gamut by using
a color mapping table created by correcting the color using a
predetermined function for a difference in the
lightness-directional dynamic range, and then correcting the color
by a combination of a three-dimensional compression of lightness,
chroma and hue and a two-dimensional shrinkage or expansion of the
lightness and chroma.
In the color mapping apparatus, the color mapping means uses the
color mapping table to change a color in the input color gamut to
one in the output color gamut.
The Invention of the present invention has proposed a
two-dimensional compression of lightness and chroma (as in the
Japanese Published Unexamined Patent Application No. 09-098298) and
a three-dimensional compression of lightness, chroma and hue (as in
the Japanese Published Unexamined Patent Application No.
08-238760). In the two-dimensional gamut compression, a
consideration is given to the gradation in a high chroma area. The
three-dimensional gamut compression permits to prevent the contrast
of a picture from being lowered and keeps the picture vivid for a
third dimension.
The present invention implements a combination of the correction of
a lightness-directional deviation due to a difference between input
and output devices (one-dimensional gamut mapping in the direction
of lightness) and the above-mentioned two methods of gamut
compression. Further, the present invention is based on these two
methods of gamut compression and a further development of the
methods. Therefore, the present invention is applicable for a gamut
mapping even when the color gamut of an output device is wider than
that of an input device.
These objects and other objects, features and advantages of the
present intention will become more apparent from the following
detailed description of the preferred embodiments of the present
invention when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates a color management system
(CMS);
FIG. 2 schematically illustrates a device profile;
FIG. 3 shows the result of comparison between a color gamut of a CG
monitor and a one of an ink-jet printer, each obtained by
integration in the direction of L* and plotting in a plane of
a*-b*;
FIG. 4 graphically shows a color gamut mapping in which only chrome
is reduced while lightness and hue are maintained constant;
FIG. 5 graphically shows a color gamut mapping with a compression
towards (L*, a*, b*)=(50, 0, 0) while hue is maintained
constant;
FIG. 6 shows forward and backward look-up tables (LUT), showing a
relationship between the LUTs;
FIG. 7 shows a comparison between measured data of a color in a CMY
space and that in an L*a*b* space;
FIG. 7A showing the measured data in the CMY space while;
FIG. 7B shows the measured data in the L*a*b* space;
FIG. 8 is a flow chart of operations up to creation of a backward
LUT based on measured data of a color;
FIG. 9 graphically shows a comparison between a hexahedron placed
as N.sup.3 pseudo measured data in a CMY space and that in an
L*a*b* space;
FIG. 9A showing the hexahedron placed in the CMY space while;
FIG. 9B shows that placed in the L*a*b* space;
FIG. 10 consisting of FIG. 10A through 10B, shows a division of a
hexahedron as N.sup.3 pseudo measured data of into five
tetrahedrons;
FIG. 11 consisting of FIG. 11A through 11B, graphically shows a
calculation to determine in which one of the tetrahedrons formed as
in FIG. 10 the L*a*b* on a grid exists;
FIG. 12 graphically shows a correction of a deviation in the
direction of lightness due to a difference from one to another
device;
FIG. 13 graphically shows a function used in correcting the
lightness-directional deviation;
FIG. 14 graphically shows a two-dimensional compression done when
an input color gamut is larger than an output color gamut;
FIG. 15 is a flow chart of operations for the two-dimensional
compression done when the input color gamut is larger than the
output color gamut as in FIG. 14;
FIG. 16 graphically shows a two-dimensional compression done when
an output color gamut is larger than an input color gamut;
FIG. 17 is a flow chart of operations for the two-dimensional
compression done when the input color gamut is larger than the
output color gamut as in FIG. 16;
FIG. 18 graphically shows a color correction by a combination of a
compression of three-dimensional levels including lightness, chroma
and hue and a compression of two-dimensional levels including
lightness and chroma;
FIG. 19 is a flow chart of operations for a two-dimensional
compression done after a three-dimensional compression;
FIG. 20 is a flow chart of operations for a two-dimensional
compression done after a three-dimensional compression;
FIG. 21 is a flow chart of operations for a three-dimensional
compression done after a two-dimensional compression; and
FIG. 22 is a flow chart of operations for a three-dimensional
compression done after a two-dimensional compression.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is used for creation of a device profile,
image transform to implement a device-independent color (DIC) and
for other purposes. The present invention will be described
concerning the creation of a device profile by way of example. The
device profile refers to a model of look-up table (LUT). However,
the gamut mapping method according to the present invention is not
limited to the device profile creation but can be used for mapping
a color which cannot be reproduced using a physical model, and so
forth.
The present invention will be described concerning the gamut
mapping method using a CMY output inkjet printer (referred to
simply as "printer" hereinunder) as an output device. More
particularly, the gamut mapping method according to the present
invention is applied for creation of a backward look-up table (LUT)
which is to be stored in a ROM (read-only memory) of the printer.
That is, when the printer prints out, on a paper, a picture
displayed on and supplied from a monitor or the like as an input
device, a controller such as a microcomputer maps color signals
according to the backward LUT stored in the ROM or the like, so
that the picture displayed on the monitor or the like is printed
out on the paper in colors within a color gamut of the printer.
To create the above-mentioned backward LUT, it is necessary to
create a forward LUT first. Then, LUTs in opposite directions
including forward and backward LUTs are created for all the input
devices as shown in FIG. 6. The forward LUT is to change a
device-dependent color signal (referred to as simply as "device
signal" hereinunder) to a device-independent color signal (referred
to simply as "chroma signal" hereinunder). The backward LUT is to
change a chroma signal to a device signal. Note that this
embodiment uses a CMY signal as a device signal and an L*a*b*
signal as a chroma signal.
To create a backward LUT used to change an L*a*b* signal to a CMY
signal, an L*a*b* signal for a CMY signal in a color space of the
CMY signal is first calculated to create a forward LUT. Then, a
value of L*a*b* signal in a backward LUT for the forward LUT and a
value of CMY signal for the L*a*b* signal are calculated by an
inverse transform. Finally, a value of CMY signal for the L*a*b*
signal not defined in the backward LUT is set using the gamut
mapping method according to the present invention.
To create the forward LUT, first a device signal is measured using
a spectroscopic colorimeter or the like. More specifically, N.sup.3
(N.times.N.times.N) color patches are disposed regularly in the CMY
space as shown in FIG. 7A. The color value (CIE/L*a*b*) of each
patch is measured by the colorimeter. As shown in FIG. 6, the value
of each component of the CMY signal and value of L*a*b*, of each
patch, are written in the CMY and L*a*b* columns, respectively, in
the forward LUT. Namely, the values of each component and L*a*b*
are measured data of each color patch supplied from the
colorimeter. Note that the color patches may be disposed in any way
but they should be disposed to fully fill the device color space.
Therefore, if the color patches are disposed not to fully fill the
device color space, a forward LUT will be created by an
interpolation based on data measured by the colorimeter.
The backward LUT can be created by inverse transform of the forward
LUT as shown in FIG. 6. More particularly, when a measured signal
is assumed as L*a*b*, for example, 0.ltoreq.L*.ltoreq.100,
-128.ltoreq.a*.ltoreq.128 and -128.ltoreq.b*.ltoreq.128 are written
in the L*a*b* column in the backward LUT as shown in FIG. 6.
Namely, an L*a*b* space thus defined is divided by M.sup.3
(M.times.M.times.M) as shown in FIG. 7B. The backward LUT is a
table in which the CMY output values on the grid are stored. For
the creation of this backward LUT, a gamut mapping has to be
done.
FIG. 8 is a flow chart of operations for the creation of a backward
LUT based on data of N.sup.3 color patches measured by the
calorimeter. As shown in FIG. 7A, the N.sup.3 color patches are
arranged regularly in the CMY space. However, the data are plotted
in the color space L*a*b*, the color patches will be disposed
irregularly as shown in FIG. 7B. To create a backward LUT, the
L*a*b* space is divided by M.sup.3 (M.times.M.times.M) and the CMY
output values on the grid are determined, as mentioned above.
However, since it will be seen from FIG. 7 that all the grids are
within the a output device color gamut, it is judged at step S1
whether the L*a*b* on the grid is within the output device color
gamut as will be described below.
First, N.sup.3 measured data are subjected to the Lagrange's
interpolation to provide their pseudo measured data. The pseudo
measured data is a hexahedron of (N-1).sup.3 as shown in FIGS. 9A
and 9B. In the CMY space, the hexahedron is a cube with no
distortion as shown in FIG. 9A. In the L*a*b* space, however, it is
a distorted hexahedron as shown in FIG. 9B.
Next, the hexahedron is virtually divided into five tetrahedrons as
shown in FIGS. 10A and 10B.
Then, it is judged whether the L*a*b* on the grid is within any of
the tetrahedrons, as will be described below.
As shown in FIGS. 11A and 11B, it is assumed that an input L*a*b*
is a point P (L*P, a*P, b*P) and the coordinate of the apex of one
tetrahedron is (L*i, a*i, b*i)(where i=0, 1, 2, 3). The point P can
be calculated using the following formula (1): ##EQU1##
If the .alpha..gtoreq.0, .beta..gtoreq.0, .lambda..gtoreq.0 and
.alpha.+.beta.+.lambda..ltoreq.1 satisfy the above formula (1), the
point P is included in the tetrahedron. Therefore, when the formula
is met in any of the tetrahedrons, the point P (L*a*b*) on the grid
is included in the color gamut. Thus, it is determined that L*a*b*
on the grid is within the output device. In this case, the
controller goes to step S2 to identify a one of the tetrahedrons in
which the point P is included, and then goes to step S3.
At step S3, the tetrahedron thus identified is subjected to linear
interpolation as in the following to calculate a device signal P
(cp, mp, yp) in the CMY space for the color signal P (L*p, a*p,
b*p) on the grid. That is, on the assumption that the device signal
at each apex of the tetrahedron identified at step S2 is (ci, mi,
yi) (where i=0, 1, 2, 3) as shown in FIGS. 11A and 11B , the device
signal P (cp, mp, yp) in the CMY space for the color signal P (L*p,
a*p, b*p) on the grid can be calculated by a linear interpolation
using the following formula (2): ##EQU2##
Therefore, in this case, the values calculated using the above
formula (2) are stated in the CMY column of the backward LUT in
FIG. 6 at step S5.
On the other hand, if the above formula is not met by any one of
the tetrahedrons defined at step S1, the color signal P (L*p, a*p,
b*p) of L*a*b* on the grid is outside the color gamut of the output
device and thus has to be subjected to a gamut mapping. Therefore,
in this case, the gamut mapping is done at step S4 and values
calculated by this gamut mapping are stated in the CMY column of
the backward LUT in FIG. 6 at next step S5.
In the gamut mapping at step S4, first a color correction is done
using a function such as an exponential function to correct a
lightness-directional difference. Then, a color correction is done
by a combination of a three-dimensional compression of lightness,
chroma and hue and a two-dimensional compression of lightness and
hue. The one-, two- and three-dimensional compressions will be
described in the following:
One-dimensional Compression: Gamut Mapping for Lightness
In the one-dimensional compression, lightness is compressed or
expanded with the chroma and hue kept constant to correct a
lightness-directional deviation due to a difference between the
output and input devices. When the maximum density of the output
device is lower than that of the input device, in other words, when
the lightness L* of the output device is higher than that of the
input device, a black compression will be caused depending a color
correction method as the case may be, resulting in a loss of the
gradation in low-lightness area. On the contrary, when the maximum
density of the output device is higher than that of the input
device (i.e., the lightness L* is lower), the color gamut of the
output device will not be used to the full extent.
To solve the above problem, it is necessary to correct a deviation
in the direction of lightness as shown in FIG. 12 by effecting some
correction. For the correction of the lightness-directional
deviation, a variety of functions can be applied as shown in FIG.
13. In this embodiment, however, an exponential function used for
correction of a knee and gamma should be used for the correction of
the lightness-directional deviation. If an exponential function is
used for the one-dimensional compression, the gamut should be
mapped to L*_out=(100-L*_min).times.(L*_in/100) .gamma.+L*min where
L*in is a lightness before being corrected, L*_in is a lightness
after being corrected and L*_out is a lightness after being
corrected. When the maximum density of the output device is lower
than that of the input device (i.e., the lightness L* is higher), a
gamma (.gamma.) value ranging from 1.0 to 1.3 is considered to be
suitable and it should be larger as the lightness-directional
deviation is larger. On the contrary, when the output device is
higher in maximum density than the input device (lightness L* is
lower), a gamma value ranging from 0.75 to 1.0 is considered to be
suitable and it should be smaller as the lightness-directional
deviation is larger. Thus, the lightness-directional deviation
between the input and output devices can be corrected to inhibit a
phenomenon such as black compression from taking place and maintain
the gradation in the low lightness area of a picture, whereby the
color gamut of the output device can be used to the full
extent.
The compression may be done using the following tristimulus
values:
where A_out=func (A_in) is a function defined by
(A_out-Amin_o)/(Amax_o-Amin_o)={(A_in-Amin_i)/(Amax_i-Amin_i)}
.gamma. where Amax_o and Amin_o, and Amax_i and Amin_i represent
maximum and minimum values of the respective signals and maximum
and minimum values of an input signal.
Also, the following compression may be done by transforming the
color signal to tristimulus values linearly variable with the human
visual characteristics:
X, Y and Z are used for tristimulus values but other tristimulus
values may be used.
Two-dimensional Compression: Gamut Mapping for Lightness and
Chroma
The gamut mapping method according to the present invention is
applicable when the color gamut of the output device is smaller
than that of the input device as well as when the color gamut of
the output device is larger than that of the input device. FIGS. 14
and 15 show a two-dimensional compression to be done when the color
gamut of the input device is larger than that of the output device,
and FIGS. 16 and 17 show a two-dimensional compression to be done
when the color gamut of the output device is larger than that of
the input device.
It should be noted that since in an operation done when the color
gamut of the output device is smaller than that of the input
device, it is expanded to that of the input device, the operation
will be referred to as "two-dimensional expansion" herein. Further,
it should be noted that since in an operation done when the color
gamut of the output device is larger than that of the input device,
it is shrunk to that of the input device, the operation will be
referred to as "two-dimensional shrinkage" herein. For this
two-dimensional expansion or shrinkage, the input gamut is divided
by four with a first straight line 1.sub.1 passing through a
minimum value point L*_min of the lightness L* in the output gamut
and a second straight line 1.sub.2 passing through a maximum value
point L*_max of the lightness L* in the output gamut, these first
and second straight lines intersecting each other at a point
(C*_th, L*_th) of the lightness value L*_th having a maximum chroma
value C*_max of the output gamut in a two-dimensional plane of the
lightness L* and chroma C* with the hue h kept constant, as shown
in FIGS. 14 and 16.
The two straight lines pass through the maximum value point L*_max
and minimum value point L*_min of the lightness L* of the output
gamut, respectively, and intersect each other at a point. The
intersecting point lies on a point (C*_th, L*_th) on the lightness
L*_th having the maximum value of the chroma C*_max.
The two straight lines can be expressed by the following
formulae:
where a.sub.1 and a.sub.2 are gradients of the two straight lines,
respectively, and a.sub.1 =(L*_th-L*_min)/C*_th and a.sub.2
=(L*_th-L*_max)/C*_th. In the above formula, L*_max and L*_min are
maximum and minimum values, respectively, of the output lightness,
C*_max is the maximum value of the output chroma and L*_th is the
lightness for the maximum value of the output chroma.
Also, C*_th is a parameter defined by C*_th=C*_max.times.K
(constant, 0.ltoreq.K.ltoreq.1).
Therefore, the input color gamut is divided by four as in the
following:
On the assumption that the value of a color picture data of an
input color signal mapped in the CIE/L*C*h color space is (L*_in,
C*_in) and the value the color picture data has after being
compressed is (L*_out, C*_out), 1=L*_in and c=C*_in are put in the
formulae for the two straight lines to discriminate the area.
If the area is determined to be the first area AR1, the output
color gamuts will be the input color gamuts, namely, L*_out=L*_in
and C*_out=C*_in as shown in FIGS. 15 and 17.
If the area is determined to be the second area AR2, expansion or
shrinkage is done on a straight line passing through the point
(L*_min, 0) and (L*_in, C*_in). On the assumption that the maximum
values of the input and output color gamuts, respectively, on the
straight line are (L*_m, C*_m) and (L*_p, C*_p), respectively, and
the value on the boundary between the first and second areas is
(L*_tmp, C*_tmp), the output gamuts will be as follows:
If the area is determined to be the third area AR3, expansion or
shrinkage is done on a straight line passing through the point
(L*_max, 0) and (L*_in, C*_in). On the assumption that the maximum
values of the input and output color gamuts, respectively, on the
straight line are (L*_m, C*_m) and (L*_p, C*_p), respectively, and
the value on the boundary between the first and second areas is
(L*_tmp, C*_tmp), the output gamuts will be as follows:
If the area is determined to be the fourth area AR4, expansion or
shrinkage is done on a straight line passing through the point
(L*_th, C*_th) and (L*_in, C*_in). On the assumption that the
maximum values of the input and output color gamuts, respectively,
on this straight line are (L*_m, C*_m) and (L*_p, C*_p),
respectively, the output gamuts will be as follows:
(L*_in>L*_th)
(L*_in<L*_th)
(L*_in=L*_th)
With the value of parameter K increased, the change in the
direction of chroma can be made smaller. By decreasing the value of
parameter K smaller, the change in the direction of lightness can
be made smaller. The optimum value for picture output is
0.5.ltoreq.K.ltoreq.1.0. The parameter K should be large for a
printer having a better reproducibility.
Three-dimensional Compression: Compression of Lightness, Chroma and
Hue
A visual difference between two colors, which is quantitatively
represented, is called a color difference .DELTA.E*.sub.ab and it
can be expressed with the following formula (3): ##EQU3##
where .DELTA.L*.sub.ab, .DELTA.C*.sub.ab and .DELTA.H*.sub.ab are
differences in lightness, chroma and hue between two colors. The
smaller the color distance .DELTA.E*.sub.ab, the smaller the visual
difference between the two colors will be.
The algorithm employed in the gamut mapping method according to the
present invention is such that the three terms (lightness, chroma
and hue differences) in the ordinary color difference formula are
weighted (the weighting factors are referred to as "coefficient of
compressibility" hereinunder) for gamut compression in a direction
in which each color difference is minimized. Namely, it is an
algorithm for such a color that when the following formula (4) is
used to estimate the color differences, the difference .DELTA.E is
minimized.
By increasing the values of the coefficients of compressibility Kl,
Kc and Kh, the compressibility for the attribute of each term is
increased. That is, by changing the coefficient of compressibility,
it is possible to find which one of the three attributes is most
important and should be compressed. By increasing the value of any
one of the three coefficients of compressibility, the compression
is made more approximately to the one-dimensional compression. By
increasing simultaneously the values of any two of the three
coefficients of compressibility, the compression can be made more
approximately to the two-dimensional compression. For example, by
increasing the coefficient Kl, the compressibility is larger in the
direction of lightness while increasing the coefficient Kc will
increase the compressibility in the direction of chroma. By
increasing simultaneously the coefficients Kl and Kc, the
compression is made more approximately to the two-dimensional
compression of lightness and chroma with the hue kept as constant
as possible. When all the coefficients of compressibility are set
to one, the compression is made as represented by the ordinary
color difference formula.
The coefficients of compressibility Kl, Kc and Kh are suitably in
the following relation:
The gamut mapping method according to the present invention is
characterized in that the one-dimensional compression for the
above-mentioned lightness-directional correction is followed
by:
(1) Three-dimensional compression and then two-dimensional
compression, or
(2) Two-dimensional compression and then three-dimensional
compression.
The gamut compressions (1) and (2) will be described below with
reference to FIG. 18. The input color gamut is a sum of the areas
A, B and C, and the output color gamut is a sum of the areas A and
B. The area D is a further area taking place in creating a LUT.
When the input device is a monitor, sRGB should preferably be
applied.
The areas in FIG. 18 are defined as follows:
Area A: Area above the aforementioned straight line 1.sub.1 and
below the second straight line 1.sub.2
Area B: An input area other than the area A and corresponding to
the output color gamut
Area C: An input area other than an area corresponding to the
output color gamut
Area D: An area other than the output and input color gamuts
The gamut mapping (1) in which the three-dimensional compression is
done and then the two-dimensional compression is done will be
described with reference to FIGS. 19 and 20. In this case, the
following two methods had provided good results for reproduction,
at a printer, of colors displayed on a monitor.
In the first one of the two methods, first the output color gamuts
in the areas C and D are three-dimensionally compressed to the area
(A+B), and then the area (B+C) is two-dimensionally compressed
(shrunk) to the area B as shown in FIG. 19.
In the second method, first the output color gamuts in the areas B,
C and D are three-dimensionally compressed to the area A and then
the area A is two-dimensionally compressed (expanded) to the area
(B+C) as shown in FIG. 20.
The gamut mapping (2) in which the two-dimensional compression is
done and then the three-dimensional compression is done will be
described with reference to FIGS. 21 and 22. In this case, the
following two methods had provided good results for reproduction,
at a printer, of colors displayed on a monitor.
In the first one of the two methods, first the area B is
two-dimensionally compressed to the area (B+C) and then the output
color gamuts in the areas C and D are three-dimensionally
compressed to the area (A+B) as shown in FIG. 21.
In the second method, first the area B is two-dimensionally
compressed (expanded) to the area (B+C) and then the output color
gamut in the area D is three-dimensionally compressed to the area
(A+B+C) as shown in FIG. 22.
The gamut mapping method having been described in the foregoing
transforms a color of a picture to a color signal for each of the
weighted three terms in the color difference formula (lightness
difference .DELTA.L*, chroma difference .DELTA.C*.sub.ab and hue
difference .DELTA.H*.sub.ab) to be minimum, thereby permitting to
fully keep the characteristics such as contrast, third dimension
and vividness of a picture. Also, the method divides a picture into
two areas of lightness and chroma with the hue kept constant and
compresses each area optimally, thereby permitting to maintain the
gradation in an area having a large chroma. Furthermore, the method
corrects a lightness-directional deviation between the input and
output devices, thereby permitting to prevent a black compression
or the like from taking place and keep the gradation of a picture
at a low lightness. Thus, the color gamut of the output device can
be used to the full extent.
Therefore, even when a color signal outside the output color gamut
smaller than the input color gamut is supplied, the gamut mapping
method according to the present invention can transform the input
color signal to the output color gamut while fully keeping the
characteristics such as contrast, third dimension and vividness of
a picture. Further, even when the output color gamut is larger than
the input color gamut, the gamut mapping method according to the
present invention can transform the input color signal to the
output color gamut while fully keeping the characteristics such as
contrast, third dimension and vividness of a picture.
As having been described in the foregoing, the present invention
provides a gamut mapping method of changing, when an output color
gamut is different from an input color gamut, the input color gamut
to the output color gamut by correcting the color with a
predetermined function for a difference in the
lightness-directional dynamic range, and correcting the color by a
combination of a three-dimensional compression of lightness, chroma
and hue and a two-dimensional shrinkage or expansion of the
lightness and chroma. Therefore, even when a color signal outside
the output color gamut smaller than the input color gamut is
supplied, the gamut mapping method according to the present
invention can transform the input color signal to the output color
gamut while fully keeping the characteristics such as contrast,
third dimension and vividness of a picture. Therefore, the present
invention permits to provide an output device such as a printer,
and so forth, capable of a natural color reproduction at different
types of devices in a picture input/output system such as DTP, and
so forth.
* * * * *